Importance of plasmalemma H +-ATPase activity for N losses from intact roots of spring wheat ( Triticum aestivum L .)

Abstract

The hypothesis of this investigation is that a substantial proportion of nitrogen (N) released by roots of entire plants into the soil are amino acids which are transported from upper plant parts to roots via phloem and from the phloem released into the root apoplast. From here amino acids may diffuse directly into the outer medium (soil or nutrient solution). Alternatively, amino acids in the root apoplast may be taken up by root cells passing the plasmalemma by H + co-transport. This membrane transport is dependent on protons which are provided by the plasmalemma H + pump (plasmalemma H + ATPase). If this concept is correct measurements which influence the activity of the plasmalemma H + pump should have an influence on the release of N from roots into the outer medium. It was shown that entire wheat plants ( Triticum aestivum L.) released almost five times more 15N by roots into the soil when treated with vanadate for 3 days than the control plants. Vanadate is a selective inhibitor of the plasmalemma H + pump. Fusicoccin, which stimulates the plasmalemma H + pump, significantly depressed the release of 15N from roots into the outer medium. Accordingly, it was shown that the H + pump was activated by fusicoccin and inhibited by vanadate. Also measurements, such as high temperature (35 °C/25 °C, day/night) and water-logging increased significantly the release of 15N by roots into the soil medium. These measurements are supposed to impair the provision of the plasmalemma H + pump with ATP. Plants treated with vanadate showed a significantly higher release of individual amino acids into the outer medium as compared with the control plants. For plants treated with fusicoccin the reverse was true. Here, the release of individual amino acids from roots into the outer medium was depressed relative to the control plants. The results obtained are consistent with the hypothesis that amino acids play a major role in N release by roots of entire plants into the outer medium. The release is a diffusion out of the root apoplast into the medium. For this diffusion the activity of the plasmalemma H + pump plays a crucial role, since it controls the uptake of amino acids from the root apoplast into adjacent root cells. With high uptake rates less amino acids are available for diffusing into the outer medium and vice versa.